Delay Alternating with Nutation for Tailored Excitation (DANTE) pulse trains are a method used for frequency-selective excitation of a narrow frequency region in high-resolution NMR spectroscopy [1]. Another important application of DANTE pulse trains is in spatial tagging of magnetic resonance (MR) images [2] where a tagging gradient is played out concurrently with a series of low flip angle hard radio frequency (RF) pulses in order to saturate 1D bands or 2D grids of tissue signal, for example in the assessment of cardiac wall motion [3].
Motion sensitivity of steady-state free precession (SSFP) sequences has long been recognized and studied [4]. Although some applications relevant to flow suppression have been proposed [5-10] the importance of SSFP mechanisms for flowing-spin signal attenuation has not been fully realized. This is likely due to the fact that standard SSFP readout imaging sequences use selective RF pulses, for which the inflow signal enhancement of flowing spins is usually much stronger than any flowing spin signal attenuation effects. Another reason is the complexity in quantification of flowing spin attenuation, given the laminar pulsatile flow patterns typical in vivo, leading to a wide distribution of spin velocities in fluids such as blood and cerebral spinal fluid (CSF). However, flowing systems with velocities below 1 mm/s have been studied extensively by Patz et al., for which a flow de-phasing parameter has been introduced to quantify the signal attenuation in SSFP [11].
Several methods have been employed in the literature for effective flowing spin suppression, also known as black blood (BB) preparation, in order to assess vessel wall and spinal cord anatomy and pathology [12-17]. Among those methods, the Double Inversion Recovery (DIR) [15] and Motion Sensitive Driven Equilibrium (MSDE) [17] techniques are the two most prominent, chosen on the basis of the quality of their flow suppression effectiveness and their image acquisition efficiency. The DIR technique is known to have reasonable flowing spin suppression [18]. However, DIR imaging acquisition efficiency is generally compromised by its requirement for single-slice sequential acquisition, due to the use of non-selective 180° pulses [17] to define the T1 null of the blood. In addition, the BB effect of DIR relies heavily on the balance between the flow velocities present and the thickness of the imaging slice. As such, it is very difficult to achieve high quality BB multi-slab 3D imaging with the DIR preparation due to the substantially increased outflow volume required for effective blood nulling compared to 2D imaging [18].
The MSDE module has been proposed as an alternative method to DIR with more robust flowing spin suppression qualities to address the difficulties of multi-slice 2D and multi-slab 3D image acquisition [17]. Sources of static signal loss are, however, inevitable, including inherent T2 decay, T1 steady state decay [19] and diffusion attenuation introduced by the MSDE preparation module. Some further sources of signal loss, such as eddy currents from the strong flow crushing gradients and imperfections in the MSDE module's 180° pulse(s) caused by B1 inhomogeneities, can also be present. Moreover, specific absorption rate (SAR) problems due to the employment of multiple 90° and 180° pulses significantly compromise the use of multi-slice MSDE preparation at high static field [17].
In general, the biggest problem of these conventional techniques is that neither of them can be adapted as an ideal module for multi-interleaved slice acquisition.
Accordingly, there is a need to address the aforementioned deficiencies and inadequacies.